Pulsed laser sources, such as Nd:YAG lasers, have been used to perform laser-based material processing for applications such as ablating, marking, engraving, micro-machining, cutting, and scribing. More recently, laser systems based on fiber gain media have been developed. In some of these fiber-based laser systems, fiber amplifiers are utilized. Some optical amplifiers and lasers utilizing a fiber gain medium are optically pumped, often by using semiconductor pump lasers. The fiber gain medium is typically made of silica glass doped with rare-earth elements, such as ytterbium or erbium. Ytterbium is used for optical amplifiers and lasers emitting in the 1020 nm-1100 nm wavelength range.
When the laser is operated in a pulse-on-demand mode, the first optical pulse in a series of pulses tends to be more powerful than the following pulses in the series. This situation is sometimes referred to as the first pulse problem and occurs because the energy stored in the laser gain medium, which is depleted significantly after the first pulse, is not fully replenished by the time the next and subsequent pulses pass through the gain medium. However, if the next pulse arrives after the gain medium is fully recovered, then another pulse similar to the first pulse will be produced. Hence, depending on the pulse repetition rate, which can be varied during operation of the laser, the energy in each pulse will generally vary as a function of the state of the gain medium. In laser processing applications, this behavior is generally undesirable because inconsistency in laser pulses will often produce correspondingly inconsistent results during processing operations.
Thus there is a need in the art for fiber-based amplifiers with repeatable output pulse characteristics independent of the pulse repetition frequency.